This invention relates to polyorganosiloxane compositions. More particularly, it relates to addition curable polyorganosiloxanes and still more particularly to materials having improved adhesion to metal and plastic substrates.
The use of polyorganosiloxanes (hereinafter sometimes "silicones") in the form of elastomeric compositions is now widespread. Elastomeric silicones include room temperature vulcanizable compositions, which cure upon standing in the presence of moisture, and addition curable compositions, which cure rapidly upon heating.
Addition curable silicones are of particular use when rapid cure is desirable or when the presence of moisture or, ultimately, of the by-products formed during the curing of room temperature vulcanizable materials is not desired. Illustrative uses are as potting compounds, in sealed environments in which water is not tolerated or in electronic applications. Curing of the addition curable silicones may be by direct heating or by an equivalent means such as ultrasound.
For such electronic applications as insulating coatings for circuit boards, elastomeric silicones in the form of conformal coatings are often preferred. Such coatings are resistant to high temperature and high humidity conditions, in addition to being electrically insulating.
Cured silicone elastomers prepared from addition curable compositions are frequently lacking in adhesion to metal and plastic substrates. Thus, improvement of their adhesion is very desirable. While the use of high curing temperatures, typically 100.degree. C. or greater, is tolerable for elastomeric coatings on metal substrates, substantially lower curing temperatures, ordinarily no higher than 85.degree. C., are frequently required in the case of resinous and especially thermoplastic substrates to avoid heat-induced distortion. The problem of low adhesion is even greater in the case of curing at such low temperatures. At room temperature (i.e., about 25.degree. C.), curing is generally so slow as to be commercially unfeasible.
The present invention includes adhesion promoting compositions for addition curable silicones which, when incorporated in said silicones, are effective with both metal and plastic substrates and, depending on molecular structure, at both low and high curing temperatures, often including room temperature. Said adhesion promoters are suitable for use in such specialized silicone compositions as conformal coatings.